The present invention relates, in general, to variable speed pulley assemblies, and more particularly relates to variable speed pulley assemblies which are coupled by a spring biasing element to effect the transmission of torque to the pulley faces and to effect biasing of the pulley faces toward each other.
Variable speed pulleys are widely used in connection with driving a variety of machine tools. Such variable speed pulleys are typically constructed by providing a pair of pulley faces that are mounted, usually by keying, to a common pulley shaft. At least one of the faces will be axially displaceable along the shaft toward the remainder of the faces, and a spring, concentrically mounted with respect to the shaft, urges the pulley faces toward each other. If desired, and without changing the basic function of the variable speed drive, both pulley faces can be movably mounted to the shaft and individually spring biased toward each other. The pulley faces are usually used to drive an endless V-belt, which in turn is mounted to a second pulley assembly.
Variable speed pulleys are especially well adapted for use in connection with driving band saws for precision cutting of metallic workpieces. In a precision band saw it is extremely important to be able to damp vibrations out of the system, since such vibrations can be reflected in poor cutting characteristics. Moreover, many precision band saws are semi- or fully automated for high production output. This automation places a premium on maintenance and on minimization of downtime that can result either from fatiguing of the saw drive train or the V-belt.
When a conventional variable speed pulley is employed to drive a precision band saw, it has been found that detrimental vibrations will be introduced into the system by the variable speed pulley. It is well known that even carefully manufactured V-belts will vary in width over the length of the belt. Such width variation is accommodated by the displaceable faces of a conventional variable speed pulley, but the constant, although slight, adjustment of the pulley faces to accommodate varying belt width will also result in a slight variation in the belt speed. The slight variation in the belt speed, in turn, is communicated back to the motor and/or gear reduction box through the pulley shaft, and the cycling of this speed variation as the pulley assembly is driven will induce vibration in the saw drive train that will be transmitted to the saw blade and adversely effect cutting.
In some instances drive train vibration will be induced by the workpiece material, configuration or manner in which it is gripped. Vibration also can result from other dynamic imbalances in the machine. Whatever the source, the effect on cutting is detrimental.
One approach which has been taken to attempt to eliminate the drive train vibration induced by variable speed pulley assemblies has been to drive the pulley assembly through a torsion spring. Thus, instead of keying the pulley faces for rotation with the pulley shaft, the pulley faces are provided on sleeves which are rotatably mounted to the pulley shaft and a torsion spring coupled to transmit rotational forces between the pulley faces and the shaft. The torsion spring also can be used to provide the double function of urging the pulley faces toward each other to firmly engage the V-belt.
Such prior art torsion spring drives of variable speed pulley assemblies have been found to have two problems. First, while the torsion spring flexes to accommodate speed changes resulting from belt width variations, the torsion spring will not completely eliminate drive train vibration. The torsion spring will flex to accommodate a speed change, but then the stored energy in the spring will cause a torque that goes beyond merely catching up to the speed differential between the shaft and the pulley faces. Thus, the torsion spring drive of pulley faces tends to cause over-correction in one direction followed by over-correction in an opposite direction, "hunting" or oscillation about the desired speed instead of correction to the desired speed.
The second problem discovered in connection with torsion spring driven variable speed pulley assemblies is fatigue of the torsion spring. The transmission of torque as well as a compressive load to the pulley faces fatigues the torsion spring and greatly decreass its useful life. Enlarging the spring to resist fatigue tends to result in a pulley assembly which is undersirably large.
Torsion spring driven, variable speed pulley assemblies have been commercially available for several years. In addition to such commercially available apparatus, the patent literature contains other examples of variable speed pulley assemblies, e.g., U.S. Pat. Nos. 2,283,392, 2,607,235, 2,699,071, 2,891,410, 2,900,834, 2,994,228 and 3,616,706.
In U.S. Pat. No. 2,607,235, instead of keying the pulley faces directly to the pulley drive shaft, the faces are free to rotate with respect to the drive shaft. Torque is transmitted to the pulley faces through the compression springs, which springs also will flex to reduce the tendency to transmit speed variations resulting from variable belt widths backwardly through the pulley shaft to the gear reduction box of the motor. Use of the compression springs, therefore, to drive the pulley faces, instead of keying the pulley faces to the shaft, is helpful in reducing overall system vibration.
In addition to the problem of hunting induced by the torsion springs, the pulley assembly of U.S. Pat. No. 2,607,235 is much more bulky than is desirable. This increase in size cannot be accommodated in some applications. Additionally, the pulley assembly of U.S. Pat. No. 2,607,235 induces belt wear by reason of the fact that the pulley faces are each independently driven and independently rotatable on the pulley shaft. Thus, variations in belt widths and loading on the pulley assembly will induce relative angular displacement between the pulley faces that causes wear, which in turn can induce further speed variation, more torsionally induced hunting and even more wear.
U.S. Pat. No. 3,616,706 discloses a variable speed pulley assembly in which mating tapered transmission surfaces couple the pulley faces to the drive shaft. While accommodating axial displacement of the pulley faces, they do not have the flexure or chushioning effect of driving through a torsion spring. Accordingly, chatter is transmitted to the drive train in a manner similar to keying of the pulley faces directly to the drive shaft.
The remaining of the above-set forth patents are directed to various compression spring assemblies and cam-acting drive systems.